For executing measurements on samples by means of magnetic resonance, one needs a radiofrequency magnetic field which, together with a constant magnetic field, excites magnetic resonance phenomena within the sample. In order to generate a radiofrequency magnetic field of utmost intensity at the location of the sample, various resonator assemblies have become known.
For measurements by means of nuclear resonance, the frequency of the magnetic field is conventionally of the order of several 100 MHz. In that case, the resonator assemblies are mostly configured as coil assemblies. For measurements by means of electron resonance, however, the frequency of the magnetic field is of the order of several 10 GHz up to several 100 GHz due to the much higher gyromagnetic ratio of electrons. In that case, the resonator assemblies are conventionally configured as hollow cavity resonators.
U.S. Pat. No. 4,437,063 discloses a probe head for measuring paramagnetic electron resonance. The probe head comprises a hollow cavity resonator being configured within a cylindrical tube section between a piston and an end of the tube closed by a coupling member. An axial bore is provided through the piston for introducing a sample. A non-axial hollow waveguide section is provided within the opposed coupling member. The oscillation mode being able to propagate within the hollow cavity for exciting electron resonance within the sample, is unable to propagate within the hollow waveguide section and within the axial bore for the sample. The prior art probe head is not intended to nor adapted for being used for the simultaneous excitation of nuclear resonance within the sample.
For hollow cavity resonators, the coupling of the microwave signal as well as the receiving of a sample is of particular importance. For coupling the microwave signal, an iris coupling is conventionally used or a coupling by means of an antenna. For very high microwave frequencies of the order of several 100 GHz, however, problems arise due to the very small dimensions of the components which basically correspond to the wavelength which, for 300 GHz, is only 1 mm.
In this connection, a particular ENDOR (Electron Nuclear Double Resonance) resonator assembly has become known from a project of Denysenkov et al. In this resonator assembly, electron resonance (ESR) at a frequency of 260 GHz and nuclear resonance (NMR) at a frequency of 400 MHz are simultaneously excited within a sample being positioned within a constant magnetic field of 9.4 T field strength. This prior art resonator assembly uses a cylindrical hollow cavity resonator for the ESR microwave field being configured by helicoidally winding a band-shaped material. The helix, thus configured, is simultaneously used as a radiofrequency coil for the irradiation of the NMR radio frequency field. Two cylindrical and metallically coated short-circuit plungers are introduced into the hollow cavity in an axial direction from opposite sides. One of the short-circuit plungers or both are configured axially displaceable for tuning the frequency. The ESR microwave field is laterally coupled by means of a coupling iris from a hollow waveguide. The coupling iris is located in the center of the helix. A liquid sample is provided within a quartz capillary being located along the axis of the hollow cavity, i.e. between the short-circuit plungers.
In this prior art assembly, the coupling of the microwave field is difficult and, for inserting a sample into the hollow cavity, it is necessary to remove one of the short-circuit plungers beforehand, provided that no central through opening is provided in one of the short-circuit plungers.
Published U.S. patent application 2007/0030005 A1 discloses a probe head for nuclear resonance measurements. The probe head is configured for executing MAS-experiments in which the sample rotates about an axis which is inclined relative to the direction of the main field by the so-called “magic angle” of 54.7°. The radio frequency field is irradiated on the sample by means of a dielectric resonator being arranged around the sample as a hollow cylinder. For exciting a second kind of nuclei, the probe head is, further, provided with a solenoid coil being arranged around the sample within the dielectric resonator. An excitation of electron resonance is not provided.
German citation DE 198 34 939 A1 describes a micro spectrometer for the ESR spectroscopy in which for frequency-tuning a H102-resonator, a dielectric bolt is screwed into the resonator.
U.S. Pat. No. 4,633,180 discloses a hollow cavity resonator being configured as a so-called split-ring-resonator, i.e. for example comprising two half-cylindrical shells being positioned mirror-symmetrically with regard to a longitudinal axis and being at a distance from each other with a gap in a circumferential direction.
U.S. published patent application 2007/0007961 A1 describes a spectrometer in which a nuclear resonance or an electron resonance is excited by irradiating a corresponding radio frequency magnetic field on the sample under investigation by means of an antenna.
U.S. Pat. No. 3,372,331 discloses a gyromagnetic spectrometer utilizing a helix as a transmitter and receiver coil for ESR signals. The helix is fed from a coaxial line, wherein the transition from the coaxial line to the helix is configured as a dielectric taper.
U.S. Pat. No. 3,122,703 discloses a hollow cavity resonator for ESR measurements in which particular portions of the resonator housing consist of a material of higher specific resistance as compared to the remaining portions and in which the inner surface of the housing is covered with a layer of diamagnetic material.
In a publication of Hessinger, D. et al., Journal of Magnetic Resonance, 147, p. 217-225 (2000), there is described a pulsed ESR spectrometer for MAS measurements utilizing a dielectric ring from sapphire as a resonator.